A mobile communication system has been developed into a high-speed, high-quality radio packet data communication system so as to outgrow an initial voice-based service and now provide a data service and a multimedia service. Recently, for supporting a high-speed, high-quality radio packet data transmission service, various mobile communication standards such as HSDPA (High Speed Downlink Packet Access), HSUPA (High Speed Uplink Packet Access), LTE (Long Term Evolution), and LTE-A (LTE Advanced) of 3GPP (3rd Generation Partnership Project), HRPD (High Rate Packet Data) of 3GPP2, and 802.16 of IEEE (Institute of Electrical and Electronics Engineers) have been developed. Particularly, the LTE system is a system developed for effectively supporting high-speed radio packet data transmission and maximizes the capacity of radio system by utilizing various radio access techniques. The LTE-A system is an evolved radio system of the LTE system and has enhanced data transmission capability in comparison with LTE.
Normally, LTE refers to base station and UE (User Equipment) (or referred to as terminal) equipment corresponding to Release 8 or 9 of the 3GPP standard group, and LTE-A refers to base station and UE equipment corresponding to Release 10 of the 3GPP standard group. Even after the standardization of LTE-A system, the 3GPP standard group is performing the standardization regarding the subsequent release based on LTE-A and having improved performance.
The existing 3G and 4G radio packet data communication systems such as HSDPA, HSUPA, HRPD, LTE/LTE-A, or the like employ an adaptive modulation and coding (AMC) method and a channel sensitive scheduling method in order to improve transmission efficiency. Using the AMC method, a transmitter may regulate the quantity of transmission data according to a channel state. Namely, if a channel state is not good, the transmitter may reduce the amount of transmission data so as to adjust a reception error possibility to a desired level. Also, if a channel state is good, the transmitter may increase the amount of transmission data so as to adjust a reception error possibility to a desired level and also effectively transmit a lot of information. Using a resource management based on the channel sensitive scheduling method, a transmitter may offer a service to selected users having a better channel state among several users. This case increases a system capacity in comparison with other case of allocating a channel to a single user for a service. Such an increase in capacity is referred to as a multi-user diversity gain. Therefore, using the AMC method and the channel sensitive scheduling method, it is possible to receive feedback of partial channel state information (CSI) from a receiver and then apply suitable modulation and coding technique at the most efficient time point.
UE may offer feedback of CSI to a base station through a periodic CSI report or an aperiodic CSI report. The periodic CSI report means that UE periodically reports CSI to a base station. The CSI may include at least one of a rank indicator (RI), a precoding matrix indicator (PMI), and a channel quality indicator (CQI). The aperiodic CSI report means that UE reports CSI to a base station at the request of the base station. Namely, if a periodic CSI report is set, UE periodically transmits the periodic CSI report without additional instructions. On the other hand, in case of an aperiodic CSI report, UE transmits once the aperiodic CSI report in response to a request of a base station and then does not perform any additional report.
In case of being used together with a multiple input multiple output (MIMO) transmission scheme, the AMC method may also include a function to determine the number of or rank of spatial layers of a transmitting signal. In this case, the AMC method does not merely consider a coding rate and a modulation type so as to determine an optimal data rate, but also considers how many layers will be used for transmission using MIMO.
FIG. 1 is a diagram illustrating time and frequency resources in LTE/LTE-A system.
Referring to FIG. 1, radio resources transmitted to UE by a base station (or referred to as ‘eNB’) are divided in the unit of resource block (RB) on the frequency axis and also divided in the unit of subframe on the time axis. In the LTE/LTE-A system, RB is normally formed of twelve subcarriers and occupies a band of 180 kHz. Meanwhile, in the LTE/LTE-A system, subframe is normally formed of fourteen orthogonal frequency division multiplexing (OFDM) symbols and occupies a time span of 1 msec. During scheduling, the LTE/LTE-A system may allocate resources in the unit of subframe on the time axis and also allocate resources in the unit of RB on the frequency axis.
FIG. 2 is a diagram illustrating radio resources in LTE/LTE-A system.
Referring to FIG. 2, a radio resource 200 is formed of one subframe on the time axis and formed of one RB on the frequency axis. Such a radio resource 200 is formed of twelve subcarriers in the frequency domain and formed of fourteen OFDM symbols in the time domain, thus having total 168 positions of unique frequency and time. In LTE/LTE-A, each unique frequency and time position shown in FIG. 2 is referred to as a resource element (RE).
Different type signals as given below may be transmitted through a radio resource shown in FIG. 2.
CRS (Cell specific Reference Signal): CRS is a reference signal transmitted for all UEs which belong to a single cell. CRS is transmitted at every subframe and used for channel estimation between a base station and UE, monitoring about availability of a radio link, fine tuning of time or frequency at baseband, and the like.
DMRS (DeModulation Reference Signal): DMRS is a reference signal transmitted for specific UE.
PDSCH (Physical Downlink Shared CHannel): PDSCH is a data channel transmitted on downlink. PDSCH is used for a base station to transmit traffic to UE. PDSCH is transmitted using RE allocated for no transmission of a reference signal in a data region shown in FIG. 2.
CSI-RS (Channel State Information Reference Signal): CSI-RS is a reference signal transmitted for UEs which belong to a single cell, and used for channel state estimation. A plurality of CSI-RSs may be transmitted for a single cell.
Other control channels (PHICH, PCFICH, PDCCH): Other control channels are used for offering control information required for UE to receive PDSCH or used for ACK/NACK transmission for operating HARQ (Hybrid Automatic Repeat reQuest) with regard to data transmission on uplink.
In order to effectively obtain a spatial diversity gain or a spatial multiplexing gain, the MIMO system transmits PDSCH after precoding. The LTE/LTE-A system determines coding technique to be applied to each transmission mode (TM) and notifies the determined TM to UE. A method which allows UE to independently perform decoding of a received signal without information about precoding from a base station is referred to as open loop MIMO transmission. On the contrary, a method in which UE receives precoding information from a base station and uses this for decoding is referred to as closed loop (CL) MIMO transmission. As one method for performing CL MIMO transmission, the LTE/LTE-A system employs TM 4 and TM 6 for simultaneously transmitting precoded PDSCH and non-precoded CRS. CRS which is different in precoding from PDSCH is transmitted. Therefore, in order for a receiver of UE to obtain a channel estimation value on the basis of CRS and then perform restoration of PDSCH, UE should know a precoding relation between CRS and PDSCH. Namely, UE can perform a receiving operation including decoding of PDSCH only if UE receives, from a base station, a notification that indicates which precoding based on CRS is applied to PDSCH. In a decoding process, UE should know a precoding form of PDSCH using CRS, and a base station notifies precoding information to UE to perform decoding. Precoding may use a plurality of precoders. In this case, it is difficult for a base station to offer information about precoding to UE. Therefore, a base station and UE have the same codebook. A base station performs precoding by using a precoder specified in this codebook and then notifies an index (or an indicator) of the precoder to UE. Additionally, UE receives PDSCH by using a precoder of the notified index. The above-mentioned codebook refers to a precoding matrix or a set of precoders. In the present LTE/LTE-A system, MIMO transmission modes based on CRS are TM 6 and TM 4. In TM 6 that supports CL spatial multiplexing of a single layer, a transmitted precoding matrix indicator (TPMI) and a PMI confirmation bit are used for a method by which a base station notifies a precoder used in precoding to UE. Since TPMI records an index (or an indicator) of a precoder used by a base station in precoding, UE can know the precoder used in precoding through TPMI. Such TPMI is formed of 4-bit length and thus can indicate up to sixteen precoders. A PMI confirmation bit is a field by which a base station instructs UE to comply with a confirmation operation. In case a confirmation bit is set to 0, UE finds a precoder with reference to TPMI. In case a confirmation bit is set to 1, UE finds a precoder with reference to the latest aperiodic CSI report without referring to TPMI. For example, if a confirmation bit is set to 1, UE may receive a signal by using a precoding matrix indicated by PMI contained in the latest aperiodic CSI report.
In TM 4 that supports CL spatial multiplexing of a multi-layer, a base station records an index of a precoder in a precoding information field and then notifies it to UE. This precoding information field is formed of six bits and can indicate four layers. In a precoding information field, there is a region capable of indicating sixteen precoder indexes with regard to each layer. Besides, a precoding information field has a region of indicating “Precoding according to the latest PMI report on PUSCH” for each layer. The details of a precoding information field are shown in Table 1 below.
TABLE 1One codeword:Two codewords:Codeword 0 enabled,Codeword 0 enabled,Codeword 1 disabledCodeword 1 enabledBit field mappedBit field mappedto indexMessageto indexMessage 04 layers: Transmit 02 layers: TPMI = 0diversity 11 layer: TPMI = 0 12 layers: TPMI = 1 21 layer: TPMI = 1. . .. . .. . .152 layers: TPMI = 15161 layer: TPMI = 15162 layers: Precodingaccording to the latest PMIreport on PUSCH using theprecoder(s) indicated by thereported PMI(s)171 layer: Precoding173 layers: TPMI = 0according to the latest PMIreport on PUSCH usingthe precoder(s) indicatedby the reported PMI(s)182 layers: TPMI = 0183 layers: TPMI = 1192 layers: TPMI = 1. . .. . .. . .. . .323 layers: TPMI = 15332 layers: TPMI = 15333 layers: Precodingaccording to the latest PMIreport on PUSCH using theprecoder(s) indicated by thereported PMI(s)342 layers: Precoding344 layers: TPMI = 0according to the latest PMIreport on PUSCH usingthe precoder(s) indicatedby the reported PMI(s)35-63reserved354 layers: TPMI = 1. . .494 layers: TPMI = 15504 layers: Precodingaccording to the latest PMIreport on PUSCH using theprecoder(s) indicated by thereported PMI(s)51-63Reserved
As mentioned above, using a conventional TPMI and a precoding information field, a base station can indicate 24 (=16) precoder indexes to UE. Therefore, if a codebook having a 2N (N>4) size is introduced so as to obtain a more improved beamforming gain than a conventional codebook, a current TPMI and a precoding information field give rise to a precoder incapable of being indicated to UE by a base station.